Fiber optic rotation sensors have been developed in many different forms. For the passive single-path multi-turn Sagnac interferometer approaches [1], the Sagnac phase shift is magnified by using many turns of optical fiber in order to increase the sensitivity of rotation sensing. For the passive resonator approaches [2], the enhancement of Sagnac phase shift is realized by recirculation of cw optical wave in relatively short fiber loop. These approaches to fiber gyroscopes have been demonstrated with high rotation sensitivity. In order to achieve wide linear dynamic range with digital rotation output from these gyroscopes, however, nonreciprocal phase shifters (such as frequency shifter [3,4], high speed phase modulator [5]) or electronic signal processors [6] are necessary. The performance of the fiber gyroscope is then limited by that of these added components.
The reentrant approach to fiber gyroscopes [7, 8], on the other hand, provides an inherent linear scale factor with frequency readout, as in ring laser gyroscopes but without associated frequency locking problem. In employs a multi-turn fiber coil where a single input pulse injected from external source recirculates around the fiber loop many times as a fraction of the optical signal is tapped out to be monitored at each turn. As the number of recirculation increases, the Sagnac phase shift induced between the counter propagating signal pulses is magnified by the number of recirculations. The system output then consists of a pulse train whose envelope is sinusoidally modulated with frequency linearly proportional to the rotation rate, as in the case of a ring laser gyroscope. This approach requires an optical amplifier in the sensing coil that can compensate for the signal loss at each recirculation, in order to obtain a large number of recirculations permitting sensitive rotation measurement.
It is well known that optical amplification in glass optical fibers can be easily achieved through Stimulated Raman scattering (SRS) process [9]. More recently, a large number of signal recirculation (of the order of 10.sup.3) was demonstrated in a fiber delay line using the SRS as an optical amplifier in the fiber circuit [10].